So, is there something on the nook that, when I select "Larger Text" on the device, that I, as the book designer, can control the percentage of biggerness that happens, and not the machine default? (the machine defaults on the devices I've seen seem to be related to the scale I mentioned above, which doesn't work for me). Lookin' for an override control, I am I am.

No. The parameters I was talking about work as follows, on most of the Nooks. There is UI to present the user with a list of font size (among a list of other choices - line spacing, justification, margins). As a UI decision there is a fixed list of choices for the user (rather than just bigger/smaller). This fixed list of sizes (hardcoded in) results in the Nook UI telling the RMSDK what the point size for 1em should be, based upon the user selection.

Also I am more sympathetic to the idea of user control than author control of the reading experience (for the things such as font, spacing, etc...), since my personal choices in reading will be different than yours